Over the past decade, investigations of nonlinear optical effects in chiral metamaterials and MSs have grown in interest[1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]} due to the possibility to enhance the chiro-optical response. Nonlinear CD, i.e. strong difference in second-harmonic generation (SHG) or third-harmonic generation (THG) conversion efficiency between LCP and RCP pump, opens to new possible applications such as nonlinear holography, nonlinear wavefront control, and chiral sensors. Although near-unity nonlinear CD has been demonstrated[1]}, [2]}, [3]}, [4]}, [5]}, the conversion efficiency and pump intensity have been limited by the employment of metallic structures. Recently, enhanced nonlinear CD with high conversion efficiency has been demonstrated in an hybrid multiple-quantum-well structure[7]}, [14]}.
However, optical absorption still limits the pump power that these structures can sustain. Albeit mainly plasmonic and hybrid structures have been investigated so far, dielectric resonators demonstrated considerably higher SHG and THG conversion efficiencies [15]}. In particular, concepts inspired by the bound state in the continuum (BIC) physics are providing a new framework to enhance nonlinear optical phenomena at the nanoscale[16]}, [17]}, [18]}, [19]}, [20]}, [21]}.
Non-plasmonic symmetry-broken structures sustaining quasi-BIC, which were recently shown to display a very important linear CD in the transmittivity[22]}, are thus promising to enable both high nonlinear CD and high conversion efficiency[4]}, [2]}, [7]}, [6]}.
